Polarization of epithelial cells is evident in two axes, in the ubiquitous apical-basolateral axis and within the plane of the epithelium as a second axis, the latter generally referred to as Planar Cell Polarity (or PCP). Examples of PCP are present in almost all organs, and in mammals, for example, most obvious in aspects of skin development with hair follicle orientation or cellular arrangements in internal organs, like the inner ear epithelium with its sensory cilia. In Drosophila, all adult cuticular structures and organs display striking PCP features, which makes the study of PCP establishment in Drosophila serve as a paradigm for the process in developmental patterning and disease in general. Analyses in Drosophila have established a conserved molecular pathway anchored around the Frizzled (Fz)/PCP core factor cassette and associated regulatory factors. This core Fz/PCP pathway and its regulatory components are conserved throughout evolution regulating many aspects of cellular polarization not only in epithelial organs, but in mammals also in directed cell migration of mesenchymal cells during gastrulation and neurulation. Although a framework of the interactions among the core Fz/PCP factors is emerging, little is known about the actual molecular mechanisms underlying their interactions. The scope of this application is to follow-up on very interesting gene identifications from a genome wide screen for novel regulators of the core PCP factors. Based on interesting preliminary data, we propose as Specific Aims to (1) address the role of the Ubiquitin-ligase activity of the Anaphase Promoting Complex (APC/C) and define which components of this complex act in PCP and how, (2) define the function of the novel ubiquitin-like modifier encoded by CG15283 in PCP establishment and how it might regulate the activity of core Fz/PCP factors or APC/C components during PCP signaling, and (3) identify the PCP specific substrates of the APC/C and define their molecular regulation by the APC/C. We have established several assays that will allow us to address these aims via a combination of functional in vivo studies in Drosophila, biochemical experiments, and cell culture analyses. As the roles of the APC/C outside cell cycle control and a function of the conserved CG15283 peptide are largely obscure or completely unknown, respectively, our application will provide first insight(s) into the mechanism(s) of these. Fz/PCP establishment has been linked to several medical abnormalities, including deafness, cancer, polycystic kidney disease, and ciliopathies. Thus the information acquired in this application will both advance our understanding of PCP regulation and organ patterning, and will also be of medical relevance in several disease contexts.